Patent GB-1120492-A (GB)
Title: Improvements in or relating to resonance transfertransmission systems
IPC: H04J3/20
Filed: 1967-01-13
Abstract: 1,120,492. Telephone exchanges. PHILIPS ELECTRONIC & ASSOCIATED INDUSTRIES Ltd. 13 Jan., 1967 [18 Jan., 1966], No. 1882/67. Heading H4K. In a resonance transfer system the charges stored in a pair of capacitors are exchanged through a transmission line including a silicon controlled rectifier. As shown, subscribers 1, 2 &c. are connected through line circuits 4, 5 &c. multiplexed on a transmission line 22 to a local connecting circuit 23. For the duration of a call a calling subscriber (say subscriber 1) is allotted a fixed time slot T1 and the called subscriber is allotted a different time slot T2. In its allotted time slot the charges on capacitors such as 10 and 11 of the associated line circuit set via the a and b wires of the subscribers set are exchanged with the charges stored on capacitors 30 and 31 respectively, in the local connecting circuit, e.g. During time slot T1 gate circuits 18 and 39 are simultaneously enabled to exchange the charge on the capacitors 10 and 30. Each gate comprises a pair of PNPN transistors 43, 44. Their emitter-collector paths are oppositely connected in parallel and initially offer a high impedance to the transmission path but in response to short gate pulses simultaneously fed to the emitter base paths via transformers 53, 57 are biased to the diode (low impedance) portion of their characteristic. The gate is thus effectively opened and passes the charges from the capacitors 10 and 30 in both directions one via each transistor and the resonance coils 14 and 35. The currents in the emitter-collector path peak above the holding current level and thus an internal base current sufficient to retain the low impedance characteristic after the gating pulse has ceased is induced. As each of these currents drops below this level the associated transistor cuts off but with sufficient delay for the current to commence to reverse its polarity (as the charges again commence to exchange). A fast diode such as 59 cuts off this reverse polarity. The power is supplied through supply bridges 24 and 25 from batteries 61, 62 in the local connecting circuit. The supply bridges are connected for speech transmission through a line 34. By measuring the potential drop across resistors 64, 66 it is possible to detect the loop condition of the subscribers lines and thus dial pulses from the subscriber. Since the polarity of the supply may be reversed it is also possible to detect direct current push-button signals generated by rectifiers and resistors. A 20 C/S ring signal of amplitude above the threshold of the gates may be passed down the transmission line. In a modification (Fig. 2, not shown), the gates are multiplexed in groups to which the fast diodes are common and the groups are multiplexed on the transmission line. In a further modification (Fig. 3, not shown), the two transistors of each gate are multiplexed on different transmission lines which pass pulses in one direction only and the resonance coils 14 and 35 are replaced by two resonance coils, one in each transmission line and located part way along its length. These resonance coils are clamped by having resistors in parallel to prevent any self-oscillation after each resonance transfer. One end of one of these resistors is connected to earth while one end of the other of these resistors is connected to negative battery in order to avoid cross-talk as a result of parallel capacity. In another modification a figure 3 type system has ten pairs of transmission wires and an array of 100 additional gates interposed at an intermediate point in the transmission line provide for cross-connection between in- and out-going wires.